Microstrip antennas are lightweight, low profile and low cost devices with a cylindrical and conformal structure suitable for replacing bulky antennas. Microstrip antennas have an inherently narrow (less than 5%) frequency bandwidth that limits more widespread usage. Numerous attempts to increase this bandwidth have met only limited success. Conventional microstrip antennas use a resonant cavity model to achieve a narrow bandwidth. Previous wide-band antennas like the horn, helix and log periodical antennas all suffer from being bulky, heavy and nonconformal. Combining the best characteristics of the microstrip and wideband antenna into one antenna would be most advantageous.
Up until now, it has not been possible to employ microstrip antennas without the disadvantages, limitations and shortcomings associated with a narrow bandwidth. By applying leaky-wave excitation to microstrip antennas, the present invention provides wideband microstrip antennas with compact size. This invention's wideband leaky-wave microstrip antenna provides a small antenna size making it ideal for antenna array elements. A leaky-wave can be excited in a waveguide of periodically placed microstrip patches on a dielectric substrate backed by a ground plane. While most transmission lines are designed to carry electromagnetic energy without much loss, a leaky-wave loses its energy along the propagation path. A simple way to produce a leaky wave is to excite the high-order modes in the transmission line. However it can be difficult to match the input impedance because the characteristic impedance and propagation constant of the leaky-wave depend on the strip width, which is the only variable in the design process at a given layer thickness with a standard substrate material. In this invention's antenna, gaps are introduced periodically in the microstrip transmission line. The resultant leaky-wave structure provides greater antenna design freedom and flexibility making it possible to design an antenna for a desired propagation constant while the input impedance is properly matched.
The compact wideband leaky-wave excitation microstrip antenna of the present invention provides the same high efficiency as in conventional microstrip antennas, with the key advantage over prior art antennas of having wide bandwidth and a similar surface area. The present invention advantageously answers the long-felt need for the low cost, compact, planar and conformal properties of microstrip material in an antenna with expanded frequency bandwidth using leaky-wave radiation.